The beginning of mobile telecommunication was dominated by a multiplicity of different incompatible systems and mobile radio networks which in most cases were set up along national and proprietary lines. Out of this situation, the definition of pan-European standards was begun during the changeover to the fully digital transmission of voice and signaling, among which standards DECT (Digital European Cordless Telephone), on the one hand, and GSM (Global System for Mobile Communications) and UMTS (Universal Mobile Telecommunication Service), on the other hand, will be compared in greater detail in the text below.
In principle, in the systems mentioned, the data to be transmitted are split into units, so-called voice or data frames, which are transmitted in succession. The size of these frames, and thus the quantity of data contained in them, and the treatment of the data within the frames for the purpose of increasing the data throughput and the data security (e.g. interleaving) are defined in the individual standards.
The DECT standard is used for supporting personal quasi-static communication services and is called cordless ISDN because of its characteristics. Due to a seamless handover, DECT is particularly well suited to dense traffic. The seamless handover in the DECT system is essentially based on the fact that a 32-kbit/s ADPCM (Adaptive Delta Pulse Code Modulation) speech codec is used which continuously supplies data. The speech frames in the ADPCM speech codec correspond to speech having the duration of one sample. At a sampling rate of, for example, 8 kHz, one frame thus corresponds to a period of 125 μs. Furthermore, the coded speech frames are not transmitted interleaved over a number of time slots.
GSM is the first digital cellular mobile radio standard which, with international roaming and ISDN capability, offers international mobility, a high data security and a great variety of data. Continuing on from this, the existing standards lead to the pan-European universal mobile radio standard UMTS. In the future UMTS standard, two modes are proposed, which are FDD (Frequency Division Duplex) and TDD (Time Division Duplex). In the FDD mode, a transmission channel is characterized by the degrees of freedom of frequency and spread-spectrum code. This is a CDMA (Code Division Multiple Access) system. The principle of CDMA consists in distinguishing between mobile radio subscribers not only by means of different frequencies but also sequences of codes. In the TDD mode, a transmission channel is defined by the degrees of freedom, frequency, time slot and spread-spectrum code. The UMTS TDD mode is called a TD/CDMA system.
In the GSM standard or in the UMTS system, too, speech codecs are used which in each case process voice or data frames having a fixed length of 20 ms. In the GSM standard or also in the TDD mode of the UMTS system, these frames are transmitted by means of the TDMA (Time Division Multiple Access) method in which the voice and data frames are transmitted distributed over a number of time slots which are produced by dividing a carrier frequency over a number of mobile stations. With a time slot period of 10 ms in the TDD mode, a voice or data frame is transmitted in two successive TDMA frames. In TDD mode, 16 time slots are provided in one TDMA frame. To be able to correct, on the one hand, random bit errors and, on the other hand, burst errors of the mobile radio channel, redundancies are added to the data to be transmitted for protecting against errors. In addition, the data are transmitted interleaved in two blocks in order to achieve further improvement with respect to disturbances such as, for example, fading. When there is interleaving, the data are not forwarded in the same order but a new temporal sorting is created which, naturally, is cancelled again at the receiving end.
In the FDD mode (CDMA), too, the data are transmitted in interleaved blocks.
The cellular structure of the entire coverage area demands that a seamless handover of a mobile station from the previous coverage area to another coverage area is established if the latter promises better transmission quality. The handover is a very critical process with respect to timing since the continuity of current calls must be ensured. It has a significant influence on the capacity and the performance of cellular networks and includes the following three phases: measurement, handover initiation, switchover to the destination base station. The continuous measurements have the purpose of detecting whether a handover is necessary. A handover algorithm makes the decision whether and when a change of transmission channel is required or appropriate dependent on various criteria such as received power, bit error rates, signal/noise ratios and distance from the current base station.
Once the handover algorithm has made a handover decision, the necessary preparations are made in the network and, in particular, the landline connection from the mobile switching center to the new base station is switched through and a new suitable transmission channel is selected. Further actions with respect to subscriber and mobility administration can be added before the final handover takes place in a third phase without regard to the interleaving of the transmission data. It is thus possible that the complete voice frame has not been received on the old transmission channel and only part of the first voice frame is received on the new transmission channel, for example only the second block of the voice frame in the UMTS TDD mode. In the worst case, a bit error rate of at least 50% arises for both voice frames due to the interleaving, with the result that repair becomes impossible even by inserted redundancies. Two successive voice frames are thus unusable, as a result of which the quality of the voice link drops since at least 40 ms of speech are missing. This loss is clearly noticeable.
The same problems occur during the transmission of packet data in packet mode or during the transmission of data with large interleaving depths. If a self-contained data packet is not completely transmitted due to a handover, important information of a larger file, which is contained in it, is lost.